The last time I gamed on a Mac (well, my hackintosh) it wasn't too bad. I was using a GTX 480, and older stuff ran well. At that time AMD wasn't supported, so even though I got a R290 in there it went a bit odd when you gamed on it.
I don't understand the concept of an Apple gaming PC though. If there's any truth to that it will be interesting to see just how hard it flops.